The river rises in the Dorset Downs at Evershot, passes through Cattistock, Maiden Newton, Dorchester, West Stafford and Woodsford.

[1] East of Dorchester the river runs over sands, clays and gravels which overlie the chalk; as the valley gradient is gentle the Frome has deposited much sediment here and thus created a broad floodplain.

Historically this contained marshes and gave the name to the Durotriges, water dwellers, the Brittonic Celtic tribe who inhabited Dorset.

Prior to the end of the last ice age, the Purbeck Hills were continuous with the Isle of Wight, and the Frome would have continued eastwards through what is now Poole Harbour and Poole Bay along the Solent, collecting the Stour–Avon, Beaulieu, and Test–Itchen–Hamble catchments, before emptying into the Channel between Bembridge and Selsey Bill.

[2] Taking the form of a channel about 1.0 m wide and 0.35 m deep cut into the underlying chalk on an artificial terrace, lined and overlain with timber, the aqueduct took water from the Frome upstream at Nunnery Mead,[3] near Notton (between modern-day Maiden Newton and Frampton) and closely followed the contours of the valley side to the southwest of the river.